Launching and receiving of surface waves



Jan. 12, 1960 GQUBAU 2,921,277

LAUNCHING AND RECEIVING OF SURFACE WAVES Filed July 13, 1956 2 Sheets-Sheet 1 w/RE MESH swam BE IN (DNTACT WITH SURFACE WAVE omaucmRz MOKABLE DIELECTRIC DISC llllllllll I INVENTOR. GEORG 1E. GDUBAU ATTORNEY n- 12, 1960 G. J. E. GOUBAU 2,921,277

LAUNCHING AND RECEIVING OF SURFACE WAVES Filed July 15, 1956 2 Sheets-Sheet 2 W5 1- mm was INVEN TOR. GEORGE J. E. sous/W ATTORNEY United States Patent LAUNCHING AND RECEIVING OF SURFACE WAVES.

Georg J. E. Goubau, Eatontown, N.J., assignor to Surface Conduction, Inc., New York, N.Y., a corporation of NewYork a Application July 13, 1956, Serial No. 597,766

13 Claims. (Cl. 333-95) This invention relates to the launching and receiving of surface waves and more specifically to' a launching or receiving device-of high efiiciency.

One of the objects of this invention is receiver of the coaxial cavity type.

Another object of the invention is to provide a cylindrical coaxial launching or receiving cavity corresponding substantially in cross section to that of the field wave of the surface wave conductor.

A further object of the invention is to use natural oscillations of a cavity resonator to produce the launching or receiving field which matches the surface wave.

A more specific object of this invention is a mode-selective coupling of a coaxial cavity to the source or receiver of the transmitted energy to produce the launching or receiving field which matches the surface wave.

Still another object of the invention is a cylindrical launcher or receiver connected over one or more coupling loops to one or more power sources of different frequencies to produce surface waves of different frequencies matching in cross section and mode with those of the common surface wave conductor connected with said launcher or receiver.

These and other objects of the invention will be more fully described in the drawings annexed herewith in which:

Fig. 1 represents a cross-sectional view of a'coaxial launcher or receiver embodying certain principles of the invention.

Fig. 2 represents a modification of Fig. 1.

Figs. 3 and 4 show examples for transparent walls which may be used instead of the transparent walls 4 and 5 of Figs. 1 and 2.

Fig. 5 represents a coaxial cavity launcher with modeselective coupling to a coaxial line.

Fig. 6 represents a modification of launcher.

Fig. 7 represents another coaxial cavity launcher.

The surface wave transmission line such as disclosed in U.S. 2,685,068 requires a launching device which forms a beam of electromagnetic wave energy of a diama launcher or a coaxial cavity eter substantially equal to that of the field of the surface wave.

match between the beam-forming device (launcher) and the surface wave. Usually a horn is used as the beamforming device, but unless the taper of the horn is not very small and the'horn not very long the field at the mouth of the horn may deviate considerably from that of. the surface wave.

The launching device described in the following provides improved efficiency in that the field distribution in the beam is made practically identical with that of the surfacewave. This is achieved-by using aresonating type of launcher. 1

As apparent from the example of the invention shown in Fig. 1 such a launcher consists of a coaxial cavity 1,

The efiiciency of the launcher depends on the field Patented Jan. 12, 1960 the center conductor 2 of which is connected to or as shown, formed of one piece with the surface wave conductor 3. Front wall 4 of cavity 1 is transparent with respect to the surface wave excited in cavity 1. Such transparency can be obtained making wall 4 of a wire mesh as shown in Fig. l or by providing properly distributed holes or slots as shown in the modification of Fig.2 at 5.

- In Fig. 3 transparency is obtained by providing segmental outputs 5 in contact with wire 2 of the surface wave conductor, and in Fig. 4 by providing circular slots 5" of appropriate width and distribution, as will be explained in the more elaborate form of Fig. 5.

The transparent wall may also be made of dielectric material with high dielectric constant or permeability.

The diameter D of the cavity'l is substantially equal to for surface wave'transmission. Since the outer con-,

ductor of this coaxial line has a very large diameter which is substantially equal to that of the surface wave field, this line can propagate several wave modes. The fundamental mode is practically identical with the surface wave propagated by the surface wave conductor. If the electrical length L of the cavity is a multiple of a half wave length of the fundamental wave mode, the cavity has a natural oscillation which consists of this wave mode multiply reflected between the front wall 4 and rear wall ,6. This particular natural oscillation is excited, for instance, by means of a coupling loop 7 connected over the inner conductor 8 of a coaxial cable 9 to a power source (not shown) of a frequency equal or close to the frequency of said particular natural oscillation. The corresponding cylindrical wave inside'the cavity 1 which is multiply reflected between the walls 4 and 6 penetrates at least partially the transparent wall 4 and continues its pathalong the open wave conductor 3.

In accordance with the invention the length L of the cavity may differ from a multiple of a half wave length of the fundamental wave mode and the cavity tuned to resonance for instance by means of a movable dielectric disc 15..

Several of such discs can beused if the cavity shallresonate in .the fundamental mode simultaneously at two or more frequencies. These frequencies can be excited in cavity 1 by means of additional coupling loops such as shown in Fig, 1 at 10 and connection of coaxial cable.

It has also been found feasible to replace the circumferential; wall 13 by longitudinal strips or conductors as schematically indicated in Fig. 2 at 14, 15, 16 and 17, circumferentially connecting the wave transparent'front wall 5 of the cavity with the reflecting rear wall 18.

instead of employing resonance to excite the fundamental wave mode in thecoaxial cavity, substantially the same result canbe'achieved by coupling the cavity to a source or receiver in such a manner that only the fundamental mode is excited. Thepartially reflecting Wall is then unnecessary and the cavity may be entirely open at the. side which faces the surface wave conductor.

Fig. 5drawn approximately to scale for a 500 me.

launcher-represents an example of a cavity with mode- 3 selective coupling to excite the fundamental mode of the coaxial cavity only. Cavity 18 is coupled to an ordinary coaxial line 19 or a tapered coaxial line 19 which is connected to the transmitter or receiver.

The coupling is achieved by means of circular slots 20, 20' and 20 of Fig, 5. in the metallic end wall v21 of cavity 18 to provide a radially symmetrical. excitation of the cavity. The same can be achieved with appropriately distributed coupling loops as shown at 7, 10, Fig. 1, but all fed from the same source. Generally the field excited in the cavity by means of circular slots consists of. the fundamental mode and higher modes of the 'EM type. The TM -modes have magnetic field lines which form circles around the center conductor. If, however, the slots are properly located the fundamental mode is only excited.

Under the assumption that the. cross-sectional dimensions of the cavity 18 are such that only the first of the higher propagating TMmnodes is Possible, the. magnetic field of this mode is zero at a certain distance r1 from the axis (see Fig. If only one. coupling slot is used, say of Fig. 5, and this slot has a medium radius. of, r1,, coaxial line 19 or 19' and cavity 18 are decoupled with regard to the higher modes. The energy which is transferred from the coaxial line.19; or 19' to thecavity 18 is completely converted into the fundamental wave mode of the cavity.

Under the assumption that the cross-sectional dimens ions of the cavity 18 allow the propagation of the first and the second higher TM -mode, the second higher TM mode has two zeros of the magnetic field, say at the distances r1 and r2 respectively. If coupling slots 20' and 20" are used (slot 20 being removed) and their radii coincide with the zeros of the magnetic field of the second higher TM -mode, this mode is not excited. In principle, each of these slots 20' or 20" alone would permit the excitation of the first higher TM -mode.

However, since the magnetic field of this mode is oppositely directed at the locations of the slots 20' and 20, in accordance with the invention the voltage across these slots can be adjusted to compensate the excitation of this mode. Thus the energy passing from line 19, 19' through slots 20 and 20" passes entirely into the fundamental mode of the cavity and is passed along the surface wave conductor.

With the use of three or more slots of appropriate radii the excitation of all the higher modes can be prevented. Generally and practically the dimensions of the cavity will allow only the propagation of the first mode, or of the first and second higher modes.

The voltages across the slots can be adjusted by means of shunt capacities, for instance by. mounting in any desired manner a metallic ring 22 which can be approached to the slot on either side of wall 21.

If the diameter of the surface wave field is very large it may be inconvenient to make the diameter of the cavity correspondingly large. In accordance with another feature of the invention the cavities of the types shown in Figs. 1, 2 or 5 may be made smaller in diameter and the field which is launched to the surface wave conductor gradually expanded by means of a horn section as shown in Fig. 6, at 23 extending from cavity 24 or by cylindrically or conically arranged tapered conductive extensions 25 of a cylindrical cavity wall, schematically shown at 26 in Fig. 7 allowing for a gradual expansion of the launching field.

The invention is not limited to the dimensions, form, shape, material and arrangement shown and described, but may be applied in any manner or form whatsoever without departing from the scope of this disclosure.

I claim:-

I. In a system for transmitting axially symmetrical transverse magnetic surface, waves along an elongated surface wave conductor, a cylindrical electromagnetic cavity enclosing coaxially an end portion of said conductor and having a diameter corresponding substantially to that of the field of said surface wave conductor as a center conductor; the remaining portion of said conductor extending from said cavity to the outside and transmitting said surface waves along the surface of said remaining portion in non-radiating mode; and said cavity being transmissive for said surface waves in the direction of said remaining portion of said conductor; and means for setting up between the ends of said cavity an. oscillating electromagnetic field of substantially the same radial field distribution as that of said surface waves.

2. System according to claim I wherein: said cavity has at the end. away from said conductor a substantially wave intransmissive side wall extending perpendicular to its axis; the other end of said cavity having a side wall which is only partially wave transmissive and is arranged adjacent to the surface wave field of the surface wave conductor.

3. System according to claim 1 wherein said cavity is adapted to resonate with a predetermined frequency and, has opposite side walls substantially perpendicular to the surface wave conductor one of said walls being only partially transmissive to said oscillations, means including at least one coupling loop'and at least one power source connected thereto of a predetermined frequency being provided for exciting between said side walls cylindrical waves multiply reflected therebetween; the diameter of said side walls being large enough to produce at least one of the natural oscillations of said cavity of a frequency substantially equal to said predetermined frequency so as to form a wave which is; substantially identical with that. of said surface. wave conductor.

4. System according to claim 3 wherein said side walls are large enough, to permit omission at least of parts of said cavity connecting said opposite side walls.

5. System according to claim 3 whereinsaid opposite side walls are electrically interconnected. by a number of elongated conductors arranged at regular distances along the circumferences of said side walls.

6. System according to claim 1 wherein the length of the cavity differs from a multiple of, halfa wave length of the fundamental mode, comprising at least one dielectric disc axially movable, within said cavity and adapted to resonate with a predetermined frequency range to resonate said cavity resonator in the fundamental mode of at least one frequency.

7'. In a system for transmitting: axially symmetrical transverse magnetic surface waves along an elongated surface wave conductor, asubstantially cylindrical electromagnetic cavity resonator enclosing coaxially an end portion of said conductor as a center conductor and corresponding in diameter substantially to that of the field of said surface. wave conductor, the remaining portion of said conductor extending from said cavity and. transmitting said surface wavesv alongv the surface. of said remaining portion substantially in non-radiating mode; and said cavity resonator being transmissivefor said surface waves in the direction of said remaining portion of said conductor; and means for setting up between the ends of. said cavity resonator an oscillating electromagnetic field of substantially the same radial field distribution as that of said surface wave including a source of transmitted energy and a mode. selective coupling connecting said source to said cavity to produce a launching field matching said surface waves.

8. System according to claim 7 wherein the front plane of said cavity resonator is open and its back plane. is coupled to a. coaxial line, the coupling surface on said back being provided with openings small against'said coupling surface providing a radially symmetrical excitation of said cavity.

9. System according; to claim 7' comprising at least one circular slot in the back of said cavity for exciting at least one of the fundamental and higher modes of the T-It Ib-type;- the slots in the back of said cavity for ex- 5 citing at least one of the fundamental and higher modes of the TM -type.

10. System according to claim 7 comprising circular slots in the back of said cavity for exciting at least one of the fundamental and higher modes of the TM -type; the cross sectional dimensions of the cavity being such that only the first of the higher propagating TM -modes is feasible and the magnetic field of said mode being substantially zero at a predetermined distance from the axis of said cavity; there being provided one coupling slot having a medium radius of said predetermined distance.

11. System according to claim 7 comprising slots in the back of said cavity for exciting at least one of the fundamental and higher modes of the TM -type; the cross sectional dimensions 'of said cavity being such that the first and the second of the higher propagating TM -modes are feasible; the second higher TM -mode having two zeros of the magnetic field at two predetermined distances from the axis of said cavity, respectively; there being provided two coupling slots having medium radii of said predetermined distances.

12. System according to claim 7 comprising conducting means movable with respect to and facing said openings to adjust the voltage across said openings.

13. In a system for receiving axially symmetrical transverse magnetic surface waves along an elongated surface wave conductor, a substantially cylindrical electromagnetic cavity resonator enclosing coaxially an end portion of said conductor as a center conductor and having a diameter corresponding substantially to that of the field of said surface wave conductor; the remaining portion of said conductor extending from said cavity to the outside and receiving said surface waves along the surface of said remaining portion in non-radiating mode; and said cavity being transmissive for said surface waves in the direction of said remaining portion of said conductor; and means for setting up between the ends of said cavity oscillating an electromagnetic field 'of substantially the same radial field distribution as that of the surface waves, including a receiver of transmitted energy and a mode selective coupling connecting said receiver to said cavity resonator.

References Cited in the file of this patent UNITED STATES PATENTS 2,129,711 Southworth Sept. 13, 1938 2,438,795 Wheeler Mar. 30, 1948 2,625,605 Chandler Jan. 13, 1953 2,653,301 Moore Sept. 22, 1953 2,685,068 Goubau July 27, 1954 

